Heart disease is the leading cause of death in the United States. A heart attack (also known as an acute myocardial infarction (AMI)) typically results from a thrombus that obstructs blood flow in one or more coronary arteries. AMI is a common and life-threatening complication of coronary heart disease. The extent of damage to the myocardium is strongly dependent upon the length of time prior to restoration of blood flow to the heart muscle.
Myocardial ischemia is an intermediate condition in coronary artery disease during which heart tissue fails to receive oxygen and other nutrients from the blood. It is typically provoked by physical activity or other causes of increased heart rate when there is coronary occlusion, that is, one or more of the coronary arteries are obstructed by atherosclerosis thus limiting the supply of blood to heart.
Patients will often experience chest discomfort (angina) when the heart muscle is experiencing ischemia. However, the ischemic episodes may also be without observable symptoms. The presence or absence of symptoms appears to be independent of the severity of the ischemia. According to the American Heart Association, millions of Americans may have silent ischemia. If even minor forms of ischemia remain untreated, affected heart tissue can eventually die, placing the patient at a high risk of having a heart attack with little or no warning. In fact, ischemia remains one of the most prevalent causes of morbidity and mortality in the developed world.
The electrocardiogram (“ECG”) is an important tool for monitoring heart activity and diagnosing heart conditions. The ECG is a recording of the electrical activity of the heart. This electrical activity causes the heart to contract. The contraction in turn causes blood to be pumped throughout the body. This electrical activity is spontaneously generated. As the cells within the heart change from a negative potential to a positive potential (depolarization), the muscles within the heart contract. Conversely, when the cells change from a positive to a negative potential (repolarization), the muscles return to their noncontracted state. The periodic contraction of the heart causes the pumping action. This spontaneous electrical activity typically occurs about once a second.
Acute myocardial infarction and ischemia may be detected from a patient's ECG, for example, by noting an ST segment shift (i.e., voltage change) over a relatively short (less than 5 minutes) period of time. U.S. Pat. No. 6,128,526 to Stadler et al. describes one type of ischemia detector that observes variation in the ST segment to identify an ischemic condition. Other ischemia detection techniques have relied upon measures of heart activity, patient workload and other factors.
By itself, an ECG may not always be sufficient for diagnosis. There are cases where an ECG may reflect the normal electrical activity of the heart, yet there is no actual pumping of blood from the heart. This condition, known as pulseless electrical activity (PEA), can be discovered if tissue perfusion is not detected even though an ECG waveform is present. Furthermore, there are certain mechanical and/or chemical changes in body physiology which typically precede or coincide with changes in the electrical activity of the heart.
For example, seconds after a coronary occlusion the potassium ion (K+) and sodium ion (NA+) concentrations in and outside cardiac myocytes are disturbed. This chemical change results in muscle contraction and relaxation dysfunction, causing mechanical changes such as a rise in the filling pressure to compensate for reduced systolic pressure. Shortly after this mechanical change, corresponding electrical activity is indicated on an ECG.
Another mechanical measure relates to heart sounds. There are four audible sounds are generated during each heartbeat that have been used to assess heart performance and augment the diagnosis of heart conditions. These heart sounds are produced by blood turbulence and vibration of cardiac structures due primarily to the closing of the valves within the heart. These four sounds are identified as S1, S2, S3, and S4. S1 is usually the loudest heart sound and is the first heart sound during ventricular contraction. S1 occurs at the beginning of ventricular systole and relates to the closure of the atrioventricular valves between the atria and the ventricles. S2 occurs at the beginning of the diastole and relates to the closing of the semilunar valves separating the aorta and pulmonary artery from the left and right ventricles, respectively. S1 and S2 can be easily heard with a stethoscope (“normal heart sounds”). S3 and S4, however, can usually not be heard in the normal heart (“abnormal heart sounds”) of a person over 40 years old. S3 occurs in the early diastolic period and is caused by the ventricular wall distending to the point it reaches its elastic limit. S4 occurs near the end of atrial contraction and is also caused by the ventricular wall distending until it reaches its elastic limit.
Heart sounds have also been used to assess the severity of important types of cardiac disease. For example, after age 40, S3 can indicate congestive heart failure, and S4 can indicate hypertension, acute myocardial infarction, or coronary artery disease.
Implantable cardiac devices, known as cardiac rhythm management devices (CRMs), that can treat heart problems are presently known and commercially available. CRMs include pacemakers (PMs) which utilize electrical impulses to regulate the beating of the heart, biventrical PMs also known as cardiac resynchronization devices (CRTs), implantable cardioverter defibrillators (ICD) for providing burst pacing pulses or a defibrillation shock to the heart when the heart is beating too fast or goes into fibrillation, and monitoring devices that use one or more physiologic sensors.
These implantable devices monitor many of the conditions that are indicative of cardiac events. However, symptoms are often highly varied. It is estimated that approximately one third of patients suffering from ischemia exhibit typical ischemic discomfort, while the remaining two thirds exhibit atypical symptoms or no symptoms at all. Similarly, classification of cardiac events is also varied. Thus, signs of trouble may be present but remain undetected by some of the common devices and detection methods for some time. Yet, it is well known that the benefits of treatment for heart conditions are greatest if administered as soon after the onset of a cardiac event as possible.
Present external automatic AMI monitoring approaches do not work well outside the hospital setting and cannot be worn for very long periods of time because of a number of factors, including the discomfort of external ECG electrodes and extensive maintenance required of these external lead systems since electrodes need to be replaced frequently leading also to poor compliance.